The objectives of this research proposal are to determine factors influencing luteinizing hormone release from bovine pituitaries in vivo and in vitro. In particular we will investigate the role of gonadal steroids in control of LH synthesis and release. Our research plan includes experiments designed to evaluate quantitative and qualitative changes in gonadal steroid output of bulls from birth to puberty. We also plan to relate changes in testicular hormone output with changes in pituitary responsiveness to gonadotropin releasing hormone in vivo and in vitro. In addition, experiments have been designed to determine the site of action of gonadal steroids in control of LH synthesis and release. Our emphasis in cows will be to determine the mechanism by which the preovulatory surge of LH occurs and the nature of negative feedback of LH release. Our proposal includes experiments which will investigate the interaction of estrogen and progesterone in control of magnitude and time of occurrence of the preovulatory LH surge. In addition, we will measure hormone changes in blood,hypothalamus and pituitary prior to and during the preovulatory LH surge in cows. We are also interested in identifying these steroids responsible for negative feedback control of LH release. Complementary studies will be carried out in vitro using bovine pituitary cell cultures; we will investigate effects of gonadal steroids and attempt to identify areas of luteinizing hormone activity in the hypothalamus of cattle.